1. Field of the Invention
This invention relates in general to overload protection circuits and in particlar to an overload protection circuit for a speaker system.
2. Description of the Prior Art
Loudspeakers are subject to two principle types of overload. The first type is thermal overload during which continuous power is applied for sufficient time to allow excessive heat to build up in the voice coil of the loudspeaker causing it to be distorted and either to short itself or to rub against the side of the magnetic gap and, thus, become opened or shorted.
The second form of overload is caused when a signal produces excessive cone excursions distorting the motor assembly in a way which prevents it from returning to its normal position. The secondary result can be shorting or opening of the voice coil or simply freezing of the motion of the cone.
Failure in a speaker can also be caused by fatigue of the suspension members but this is normally the result of long term usage rather than accidental overload.
Since thermal overload is related to energy input rather than power input, it is desirable that any sensing circuit integrate the input power so that it is energized only when the energy input exceeds the energy output by a reference amount.
However, since excursion failure occurs at low frequencies and is a function of instantaneous power input, the sensing circuit must also be energized by excess intantaneous poor at low frequencies.